The invention is based on a surge arrester as claimed in the preamble of patent claim 1.
The preamble of patent claim 1 of the invention refers to a prior art as is known from DE 19545505 C1. In the case of the surge arrester described there, for low-voltage systems, a varistor is thermally conductively connected to a thermal disconnection device comprising a fuse strip and a thermal release with a eutectic fuse alloy, and this results in the thermal release being disconnected, and a damage indicator being initiated by spring force, if the varistor is unacceptably heated. In this case, the fuse strip, which is resistant to surge currents, is arranged with weak points within a fuse housing and melts in the event of a release, overcoming the fastening of the damage indicator within the fuse housing. The thermal release, which is arranged outside the fuse housing, comprises an identifying signaling wire which keeps the damage indicator in the xe2x80x9cno indicationxe2x80x9d position and is broken when the fuse strip melts, or is melted by the short-circuit current. This surge arrester is relatively complex to produce.
The invention, as it is defined in patent claim 1, achieves the object of specifying a surge arrester of the type mentioned initially which can be produced easily and cost-effectively and is nevertheless distinguished by excellent operating characteristics.
In the surge arrester according to the invention, the varistor contains a non-metallized active part and two elastic contact elements, which are pressed against the end faces of the active part, forming an electrical contact and are preferably formed from graphite. These measures result in good electrical power transfer with a largely uniform current density from the active part to the rigid contact elements which produce the contact pressure, and these contact elements are electrically conductively connected to the electrical power connections of the surge arrester. Even if the end surfaces of the active part are not completely planar with respect to one another, but, possibly, are slightly curved in the concave or convex sense or are inclined at a small angle to one another, an excellent operating behavior is thus always achieved.
For certain embodiments of the surge arrester, it is sufficient for the active part not to be passivated at the edge.
A surge arrester having good long-term stability is obtained if the active part of the varistor is passivated at the edge, as can be achieved by the varistor having electrical insulation applied to its edge, said electrical insulation being composed of glass, epoxy varnish or a rubber or silicone ring drawn over the varistor edge. If the passivation has a ring and this ring is higher than the active part and/or the ring has an embossed profile, then it can also be used for positioning of the contact elements, which are designed to be elastic.
If the surge arrester according to the invention contains an electrical power interrupter which is in the form of a melting connection, and a damage indicating apparatus to which prestressing is applied when the electrical power interrupter is closed and which is visible once the electrical power interrupter is open, then it can be recommended that the melting connection be formed into an element of the electrical power interrupter which is in the form of a disc and is pressed against one of the two elastic contact elements of the varistor forming the contact force, since this then ensures good heat transfer from the varistor to the melting connection, and thus also reliable tripping of the electrical power interrupter.
Extremely cost-effective manufacture of a surge arrester according to the invention can be achieved if the arrester housing is predominantly axially symmetrical and has two housing parts which are connected to one another with a force fit forming the contact pressure and the prestress, in which an opening for the first electrical power connection and a part of the damage indicating apparatus which contains a signaling surface to pass through is formed in a first housing part, and an opening for the second electrical power connection to pass through is formed in a second housing part. During the manufacture of this arrester, only prefabricated parts, such as a connecting fitting or a component which contains this connecting fitting, a damage indicating apparatus and a prestressed electrical power interrupter, one of the electrical contact elements, the active part, a further one of the elastic contact elements, an electrical power transmission element which may be connected to this contact element, and a compression spring which produces a contact force are inserted into a predominantly tubular housing part, and the inserted parts are fixed, forming the contact force, by snapping them into a housing part which is in the form of a cap and contains a second connecting fitting.
The surge arrester formed in such a way can thus be manufactured very quickly and using extremely simple means, in particular even locally.